Metal oxide-hydrogen batteries, such as nickel oxide-hydrogen batteries, have seen wide use in aerospace applications. Batteries of this type are rechargeable and have an extremely long cycle life and provide a high uniform output during the entire discharge cycle.
A nickel oxide-hydrogen battery has a further advantage in that the pressure in the battery can be used as a quantitative indication of the state of charge. In addition, batteries of this type have an inherent overcharge protection.
In the typical nickel oxide-hydrogen battery, the positive electrodes are generally in the form of flat porous sintered nickel plaques, impregnated with nickel hydroxide, while the negative electrodes are formed from a thin fine mesh nickel screen having a catalyst, such as platinum, bonded to one surface of the screen through a hydrophobic polymeric material, such as polytetrafluoroethylene. On discharge of the battery, hydrogen gas diffuses through the electrolyte surrounding the catalyst surfaces of the negative plates and becomes disassociated by the catalyst to the monatomic form. The monatomic hydrogen is ionized and combines with hydroxyl ions to form water with an electron being released in the process of forming each hydrogen ion.
In addition, hydroxyl ions are formed at the positive electrode by the reaction of water with the available oxygen component of the nickel oxide. As a result of these reactions, an electron current is produced in the exterior circuit.
On recharging the reaction is reversed, with the recharging being characterized by the regeneration of hydrogen gas at the negative electrode and the reoxydation of the nickel hydroxide at the positive electrode.
Due to the gas pressures involved, the nickel oxide-hydrogen battery is contained within an outer pressure vessel.
The typical nickel oxide-hydrogen battery consists of a number of modules, each module preferably composed of a pair of back-to-back positive electrodes spaced apart by a separator layer, along with a negative electrode disposed adjacent and separated from each positive electrode. In some instances, each module is contained within a separate small pressure vessel and the modules are then connected externally in series or parallel patterns to provide the desired voltage or amperage output. In other instances, a group of modules may be contained within a single vessel and installations of this type are commonly referred to as "CPV" (common pressure vessel). In a typical CPV installation, a group of modules, which are generally rectangular in configuration, are mounted within a cylindrical pressure vessel. With this construction, there is a substantial gap or space between the sides of the battery and the inner wall of the vessel. With a nickel oxide-hydrogen battery, heat is generated in both the charging and discharging cycles and it is important that heat be transferred to the outer vessel and hence to the surrounding atmosphere. Due to the spacing of the polyhedral battery from the cylindrical vessel, heat transfer to the vessel is minimal, with the result that the performance of the battery can be adversely effected.